1. Field of the Invention
The present invention relates to an aqueous pressure-sensitive adhesive composition containing an acrylic polymer as a main component, and a pressure-sensitive adhesive sheet using such composition.
The present application claims a priority based on Japanese Patent Application No. 2006-212405 filed on Aug. 3, 2006, the entire contents of which are incorporated herein by reference.
2. Description of the Related Art
An aqueous dispersion type (aqueous) pressure-sensitive adhesive composition in which an acrylic polymer is dispersed in water uses water as the dispersion medium, and therefore is desirable from the standpoint of environmental health as compared with a solvent-based pressure-sensitive adhesive composition. In addition, such acrylic aqueous pressure-sensitive adhesive composition has an advantage of easily forming a pressure-sensitive adhesive layer having more excellent solvent resistance and a pressure-sensitive adhesive sheet having such pressure-sensitive adhesive layer.
Under these circumstances, acrylic aqueous pressure-sensitive adhesive compositions that allow the formation of higher-performance pressure-sensitive adhesive sheets are in demand. It would be useful to provide an acrylic aqueous pressure-sensitive adhesive composition suitable for a pressure-sensitive adhesive sheet, which may exhibit a plurality of properties such as adhesive strength (pressure-sensitive adhesive force), cohesive strength (in particular, cohesive strength under high temperature conditions) at a higher level in a well-balanced manner, and for fabricating such pressure-sensitive adhesive sheet.
As it has been known, one methodology to enhance the cohesive strength of a pressure-sensitive adhesive layer is to blend a crosslinking agent with the pressure-sensitive adhesive composition. For example, Japanese Patent Application Publication No. 2003-096420 discloses an emulsion-type pressure-sensitive adhesive composition wherein a crosslinking agent selected from carbodiimide compounds, oxazoline compounds, aziridine compounds, hydrazine compounds and the like is blended with an acrylic resin emulsion obtained by two-step polymerization. Japanese Patent Application Publication No. 2003-327933 discloses an acrylic emulsion-type pressure-sensitive adhesive wherein a hydrazine compound serving as a crosslinking agent is blended with an acrylic resin emulsion having a high gel fraction. In all the examples disclosed in these documents, a single (one kind of) crosslinking agent is blended with the acrylic resin emulsion. Japanese Patent Application Publication No. 2005-113016 is related to a tackifier resin emulsion which is a water-dispersed mixture of a tackifier resin and a crosslinking agent capable of reacting with the tackifier resin.
However, the conventional techniques to enhance the cohesive strength by blending a crosslinking agent are likely to significantly degrade other properties (e.g., pressure-sensitive adhesive force) in exchange for the enhanced cohesive strength. Here, even when the cohesive strength is attempted to be balanced with the pressure-sensitive adhesive force or the like by simply increasing/decreasing the amount of the crosslinking agent used, it is difficult to satisfy these properties at the same time and at a high level.